The present invention relates to a method, a system and an apparatus for echo suppression in telephony, in which part of the conversation is transmitted over the Internet.
Data communication over the Internet is a technology that has been steadily growing, with respect to the number of users, over the last years. A normal residential user blocks the ordinary telephone line when connected to the Internet since normally a modem is used to connect to the Internet through a service provider. This may cause problems, for example, when waiting for a telephone call.
This has been solved in prior art by introducing a port between the Internet and the Public Switched Telephone Network (PSTN). By introducing such a port a user can, while connected to the Internet through a modem on his ordinary telephone line, answer incoming calls and initiate outgoing calls. The technology functions by forwarding any incoming calls to said port. The port contacts a client software at the user""s site through the Internet. The client informs the user that there is an incoming call waiting. The user can choose one of a number of options, among others to receive the call. If the user chooses to receive the call, the port functions as a converter between the speech format in the telephone network and the Internet. The method is described in detail in the Swedish patent application SE-96039326.
Swedish patent application SE 9700873 discloses how crosstalk between the microphone and the loudspeaker on the Internet side can be minimized.
Echo is caused in the conventional telephone network, for example, if the impedance is not well enough adapted between the 4-wire and the 2-wire transfer. This problem is solved in the conventional telephone network by means of relatively expensive equipment and methods with high requirements on processing capacity. The methods involve, for example, filtering the sound, which requires relatively high processing capacity. To minimize the cost for the customer, of course, it is interesting to find alternative inexpensive and effective solutions. This is true in particular for the exploding Internet market in which it is important for the Internet Service Providers (ISP) to present new functions fast and where, for example, the speech quality is not as crucial.
On the Internet side no echo arises since all transmission is digital. If the user is using a headset with earphones and a microphone close to the mouth no, or very little, crosstalk is caused. Thus, no echo arises on the Internet side.
The present invention attacks the problem of how, in a cost efficient way, and utilising a minimum of processing capacity, the echo arising in the PSTN network in a call connected at least in part through a digitally based network can be minimized, while still maintaining a high quality.
Thus a purpose of the present invention is to minimize, in a cost efficient way, the echo, while minimizing the need for processing capacity.
The above problem of minimizing an echo arising in an analogue part of a connection is solved in that a switch can stop the sound from passing from the analogue to the digital part of the network.
More specifically the above problem is solved by calculating, for each Pulse Code Modulation (PCM) frame from the digital to the analogue side the energy in the frame a background energy level, an average energy level over a relatively long period and a value for the energy compared to the background energy. These paramneters are fed to a control switching function. Corresponding values are also calculated for PCM frames from the analogue to the digital side. These frames are also used by the control switching function. In dependence of the above mentioned values the control switching function decides if PCM frames from the analogue side are to be forwarded with the received data or instead be filled with zeros or background noise. PCM frames from the digital to the analogue side are not modified.
An advantage of the present invention is that the processor capacity required for the echo suppression is a fraction of that needed for a conventional echo suppression. Measurements show that in a Digital Signal Processing (DSP) board of the brand Antares from the company DLILOGIC in the USA echo suppression requires approximately 2% of the processing capacity while echo suppression according to conventional methods requires approximately 50% of the processing capacity. This means that a speech encoder, a speech decoder and echo suppression for one channel can be implemented on the same DSP board, which is not possible with conventional techniques.
Another advantage of the present invention is that it is inexpensive to implement.
Yet another advantage of said invention is that the echo suppression according to said method results in a better quality than performing echo suppression at the site of the user on the digital side. The Swedish paten SE9700873-4 describes a method for performing echo suppression at the site of the user at the digital site. If there is a risk of significant crosstalk between the microphone and the loudspeaker for the user on the digital side it may, however, be necessary to use the invention according to SE9700873-4. SE9700873-4 and the above mentioned invention complement each other in the sense that one or the other method may be used in dependence of the equipment used by the user on the digital side.
Another advantage is that the subscriber on the analogue side can participate in the conversation by talking louder than the user on the digital side.